Communication systems are known to comprise a plurality of communication units, repeaters, and dispatcher stations. The repeaters and dispatcher stations are physically connected together via a data link, such as a local area network (LAN). The communication units may either be digital communication units (transceive digital information) or analog communication units (transceive analog information). The digital communication units, because they transmit digital information, are able to communicate on a digital infrastructure directly, while analog communication units, because they transmit analog information, are incompatible with a digital infrastructure. For this reason, an analog communication unit cannot function in a digital infrastructure unless the received analog information is digitized before placement on the LAN and then decoded back to an analog signal before transmission to a receiving analog communication unit.
One implementation, which permits analog communication units to operate on a digital infrastructure, converts analog signals into a selected digital coding format. The selected digital coding format is typically based on the bandwidth of the digital infrastructure. For example, the selected digital coding format may be 4.8 kbps vector sum excitation linear predictive (VSELP), a Motorola acronym, or 64 kbps pulse coded modulation, (PCM). In general, the more bits used in the digital coding process, the more accurately the analog information can be recaptured. Conversely, the less bits used to digitally code the analog information, the less accurately the analog information can be recaptured. Thus, the 4.8 kbps VSELP digital coding may be fine for strong analog signals (high signal strength or low signal to noise), but may produce distorted representations of weak analog signals (low signal strength or high signal to noise). Consequently, the quality of the digitally coded analog signal may be unacceptable. In contrast, the 64 kbps PCM digital coding provides acceptable digital coding of weak analog signals and strong analog signals but requires substantial infrastructure bandwidth.
While the above works well when the signal quality remains relatively constant, in practice the signal quality tends to vary.
Therefore a need exists for a method of handling analog communication units within a digital infrastructure that adaptively digitally codes received analog signals based on signal strength and minimizing infrastructure bandwidth requirements.